1. Field of the Invention
The present invention relates to a detector for detecting an air-fuel ratio, or a ratio of air to fuel in an internal combustion engine or the like, and a method of measuring an air-fuel ratio.
2. Description of the Prior Art
It is the current practice to detect oxygen density in an exhaust gas emitted from an internal combustion engine on an automobile or the like and control the amounts of air and fuel to be supplied to the internal combustion engine based on the detected oxygen density value, thereby reducing harmful components in the exhaust gas.
Air-fuel ratio detectors (oxygen sensors) presently available for internal combustion engines for automobiles for detecting oxygen densities operate on the principle of an oxygen concentration cell. This type of air-fuel ratio detector is capable of detecting a stoichiometric air-fuel ratio (A/F=14.6) because of its characteristics. However, such cannot detect air-fuel ratios in other ranges, that is, in a lean range in which the air-fuel ratio is higher than the stoichiometric air-fuel ratio or in a rich range in which the air-fuel ratio is lower than the stoichiometric air-fuel ratio. When a voltage is applied between gas-permeable thin-film electrodes attached to a solid electrolyte cell permeable to oxygen ions, oxygen ions pass through the cell from the cathode to the anode, and with the passing of the oxygen ions an electric current flows between the electrodes. If the quantity of oxygen ions which are to pass though the cell is limited, then the current is not increased beyond a certain value even when an applied voltage is increased. Utilizing such phenomenon, there has been developed a limited-current oxygen sensor for detecting oxygen densities with a view to detecting air-fuel ratios in the lean range. Since the limited-current oxygen sensor is capable of detecting air-fuel ratios in the lean range only, it is called a "lean sensor", and is almost incapable of detecting air-fuel ratios in the rich range.
While an automobile is running under ordinary conditions, it is preferable that it be driven in the lean range wherein the air-fuel mixture is leaner. When the engine is required to produce a higher power output such as in the case of running up a slope, the automobile is to be preferably driven in the rich range. Therefore, there has been a demand for a detector capable of detecting an air-fuel ratio of from the rich range to the lean range.